The goal of this research is to use an asbestos aerosol to investigate the performance of respirators certified for use with asbestos. A representative sample of certified air-purifying respirators will be tested. Since asbestos is a very hazardous material, and since such respirators are widely used to protect workers from asbestos exposure, it is very important that certified respirators perform as expected. Unfortunately, the present NIOSH certification procedure does not require that respirators be tested against asbestos. It is the goal of this research to investigate the significance of this and other possible deficiencies in the certification procedure. The performance of the respirators against asbestos will be compared to that for silica, the standard dust used in the NIOSH certification procedure. Respirator penetration will be measured under steady flow conditions, as is done in the certification test, and under cyclic flow, to simulate actual use conditions. Three replicate tests will be performed for each combination of test aerosol and flow pattern, so that twelve tests will be performed for each respirator. The measurements will be analyzed by standard statistical techniques, including analysis of variance, to give the effects on performance of aerosol type, flow pattern, and respirator model. In addition, exploratory tests will be performed to assess the effects of high dust loading and elevated temperature and humidity on respirator performance, to compare penetration as measured by phase contrast and scanning electron microscopy, and to compare the performance of powered and non-powered air-purifying respirators.